1. Field of the Invention
The present invention relates to an image forming apparatus, which supplies a protective agent to an image carrier and uses a voltage including an AC bias for transferring a toner image formed on the image carrier to a recording member.
2. Description of the Related Art
Typical examples of an image forming apparatus are an electrographic copier, a FAX, a printer, and an MFP in combination with these multi-functions. Japanese Patent Application Laid-open No. 2006-267486 discloses a known technique for transferring the toner image on the surface of the image carrier toward the recording member which has been put in the transfer nip. The image forming apparatus of Japanese Patent Application Laid-open No. 2006-267486 forms a toner image on the surface of the drum-like photosensitive element through a known electrophotography process, makes the photosensitive element in contact with an intermediate transfer belt as an intermediate transfer body with an endless loop form to form a primary transfer nip, and primarily transfers the toner image on the photosensitive element to the intermediate transfer belt in the primary transfer nip. The intermediate transfer belt is designed to be in contact with the secondary transfer roller as a nip forming member from the outside to form a secondary transfer nip. A transfer facing roller is arranged inside the loop of the belt, and the intermediate transfer belt is put between the secondary transfer facing roller and the secondary transfer roller. Ground connection is made on the secondary transfer facing roller inside the loop, and a secondary transfer bias is applied to the secondary transfer roller outside the loop. As a result, a secondary transfer field is formed for electrostatically moving the toner image from the secondary transfer facing roller to the secondary transfer roller, between the secondary transfer facing roller and the secondary transfer roller. The toner image on the intermediate transfer belt is secondarily transferred by the effects of the secondary transfer field or the nip pressure, onto the recording member sent into the secondary transfer nip at a timing for synchronizing with the toner image on the intermediate transfer belt.
In this configuration, as a recording member, if a sheet of paper (for example, Japanese paper) with a large uneven surface is used, a gray-scale pattern is likely to appear in the image in accordance with the surface irregularities. This gray-scale pattern occurs as a result that the image density in the concave portion is lower than that in the convex portion, because a sufficient amount of toner is not transferred to the concave portion in the surface of the paper. In the image forming apparatus of Japanese Patent Application Laid-open No. 2006-267486, as a secondary transfer bias, a superimposed bias on which a DC voltage is superimposed on an AC voltage is applied, instead of a bias including only a DC voltage. In Japanese Patent Application Laid-open No. 2006-267486, by applying this secondary transfer bias, occurrence of a gray-scale pattern is restrained, as compared to a case in which a secondary transfer bias including only a DC voltage is applied.
In the configuration of Japanese Patent Application Laid-open No. 2006-267486, the present inventors of the invention have found that the cleaning performance is degraded in the intermediate transfer belt or the secondary transfer roller. This phenomenon occurs in the photosensitive element cleaning having a charging step. This can be considered due to deterioration of the image carrier, the charging member, and the cleaning member. This deterioration results from occurrence of an electrical stress in a charging step by a charging unit for electrically charging the photosensitive element.
To solve this problem, many proposals have been presented on a supplying method and a film-forming method, for various lubricants and lubricant components, to reduce the deterioration of the image carrier, the charging member, and the cleaning member.
For example, to extend the life of the photosensitive element as the image carrier and the cleaning blade, Japanese Patent 51-22380 suggests a technique for supplying a solid lubricant agent mainly including zinc stearate onto the surface of the photosensitive element and forming a lubricant film on the surface of the photosensitive element. This results in suppressing the abrasion on the surface of the photosensitive element and extending the life of the image carrier. However, in Japanese Patent 51-22380, it is obvious that metal salt of fatty acid (representatively, zinc stearate) loses its lubricity in the early stage, by the effect of the discharge performed in the vicinity of the image carrier during the charging step. As a result, the lubricity between the cleaning blade and the image carrier is lowered, and the toner passes therethrough, resulting in a poor image.
To solve this problem, Japanese Patent Application Laid-open No. 2006-350240 suggests a technique for applying a protective agent with a compound of metal salt of fatty acid and boron nitride to the image carrier. In this structure, even with the effect of the discharge performed in the vicinity of the image carrier during the charging step, the lubricity between the cleaning blade and the image carrier is maintained, thus enabling to prevent passing of the toner.
As disclosed in Japanese Patent Application Laid-open No. 2006-267486, applying of the AC field to the transfer nip is to improve the transferability of toner to a recording member with surface irregularities. In many cases, only a DC electric field is applied. Thus, as disclosed in Japanese Patent Application Laid-open No. 2006-350240, even if the protective agent with a compound of metal salt of fatty acid and boron nitride is applied to the intermediate transfer belt, no particular effect is attained in the normal image formation with application of only a DC electric field, in spite of the high cost.
Therefore, there is a need for a image forming apparatus capable of improving cleaning performance of an intermediate transfer body, while attaining an image of stable density.